Vision tests check many different functions of the eye. Your doctor will let you know if your eyesight is normal or if it is better or worse than normal. The doctor may also be able to tell you why you have a vision problem.
Visual acuity testing The visual acuity score compares your distance vision with that of people who have normal vision, using an eye chart. Each eye's score is expressed as two numbers, such as 20/20 (6/6) or 20/100 (6/30). The first number is the distance you stand from the chart, usually 20 ft (6 m) when using a typical wall chart. The second number is the distance from which people with normal eyesight can read the same line on the eye chart.
20/20 (6/6) vision is considered normal. A person with 20/20 vision can see at 20 ft (6 m) what people with normal vision can see at this distance.
When the second number is smaller than the first number, the person's vision is better than normal. For instance, a person with 20/10 (6/3) vision can see from 20 ft (6 m) what people with normal vision can see from 10 ft (3 m). When the second number is larger than the first number, the person's distance vision is worse than normal. A person with 20/200 (6/60) vision or less in their best eye when wearing corrective lenses is considered legally blind. Your doctor will also tell you if you have reduced near vision .
Refraction The doctor tests your eyes with different lenses until the lens that corrects your vision the best (sometimes better than 20/20 or 6/6) is found. The result of a refraction test determines your prescription eyeglass or contact lens strength.
Visual field test Normally, a person's visual field forms a rough circle with a natural blind spot. If your vision is normal, you should be able to see objects clearly throughout the entire visual field except for the area with the natural blind spot. When you are using both eyes to see, the blind spots do not interfere with your vision.
You may have vision loss in certain areas of the visual field if you are not able to see:
Test objects during tangent screen testing. Movements or light flashes during perimetry testing. Abnormal results during Amsler grid (Figure 2) testing include:
Not being able to see the black dot at the centre of the grid. Not being able to see all four edges of the grid. Having blank spots or dark spots on the grid (other than the black dot at the centre). Seeing lines that look wavy or curved. Gaps in different parts of the visual field may have many causes, including eye diseases (such as glaucoma and macular degeneration) or nervous system problems (such as stroke). If results on any of the visual field tests are abnormal, you will need further tests to determine the cause.
If visual field tests show that a person has a visual field that is 20 degrees or less, even with the help of glasses or contact lenses, that person is considered legally blind.
Colour vision test People who have normal colour vision are able to distinguish the coloured numbers, symbols, or paths from the background of coloured dots.
If you are not able to distinguish some or all of the coloured patterns from the background, you may have a colour vision problem. You may be able to pick out some patterns of colours but not others. Or you may be able to pick out patterns that are different from a person with normal vision, depending on what type of colour vision problem you have.
Many conditions can change your vision test results. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
Figure 2 - Amsler grid One of the easiest ways to screen for age-related macular degeneration (AMD) is to use an Amsler grid. An Amsler grid is a chart with lines and a dot at the centre. Your doctor can give you one to use at home.
If you have dry AMD, check your vision in each eye every day or as often as your doctor advises. It only takes a few seconds.
Cover one eye. Then, using your other eye, look at the first grid. Repeat with the opposite eye covered. If you have macular degeneration, the lines may appear wavy or curved, or you may see a blank or dark area in the middle of the grid.
In the second grid, the lines around the centre are wavy and distorted. This is one example of how the straight lines in the grid above might appear to someone with macular degeneration.
The distortion or blind spot (scotoma) you actually see may be different from what is shown in the grid picture.
Current as of: July 31, 2024
Author: Ignite Healthwise, LLC StaffClinical Review Board
